Female transfer in primates

Abstract

Intergroup transfer by males is nearly universal among social primates. Furthermore, among the most frequently studied monkeys - savanna baboons and Japanese and rhesus macaques - females typically remain in their natal groups, so troops are composed of related matrilines. These facts strongly support two major theories: 1) that kin selection is a powerful force in patterning sociality (if one is to live in a group, one should prefer a group of one's relatives), and 2) that the ultimate explanation for intergroup transfer is the avoidance of inbreeding depression (though both sexes would prefer to live with kin, one sex has to disperse to avoid inbreeding and for a variety of reasons the losing sex is generally male). Substantial rates of transfer by females in social species with routine male transfer would cast doubt on both ideas. In fact, evidence reviewed here indicates that female transfer is not unusual and among folivorous primates (e.g., _Alouatta_, the Colobinae) it seems to be routine. In addition to casting doubt on the demographic significance of inbreeding avoidance and favoring mutualistic and/or game theory interpretations of behavior over nepotistic ones, this finding supports the hypothesis that predator detection is the primary selective pressure favoring sociality for many primates. Finally, while female bonding [_sensu_ Wrangham, R. W. (1980), _Behaviour_ 75: 262-299] among primates appears to be less common than generally believed, the observed correlation between female transfer and morphological adaptations to folivory provides empirical support for Wrangham's model for the evolution of female-bonded groups.